Polyesters such as, for example, polyethylene terephthalate, polytrimethylene terephthalate and polybutylene terephthalate, are a class of important industrial polymers. They are widely used in thermoplastic fibers, films, and molding applications.
Polyesters can be produced by esterification of an acid such as terephthalic acid (TPA) with a glycol followed by polycondensation. A catalyst is used to catalyze the polycondensation and may be used to catalyze the esterification.
Antimony is often used as a catalyst for the esterification and polycondensation reaction. However, antimony-based catalysts are coming under increased environmental pressure and regulatory control, especially in food contact and fiber applications. Antimony-based catalysts may also cause problems with grey discoloration. Tin compounds can also be used in the esterification and polycondensation reactions. However, tin-based catalysts have similar toxicity and regulatory concerns.
Titanium-based catalysts, either alone or in combination with other compounds, have been described for use in the preparation of polyesters in U.S. Pat. Nos. 4,482,700, 4,131,601, 5,302,690, 5,744,571, 5,905,136, and in WO 97/45470. U.S. Patent Publication 2005/0009687 describes the use of titanium alkoxide catalyst for the polymerization of cyclic esters in particular. There has been some concern that titanium catalysts, if used in the esterification and polycondensation reactions, tend to hydrolyze on contact with water forming glycol-insoluble oligomeric species, which lose catalytic activity, as described in U.S. 2005/0215425. Esters and polyesters produced using certain titanium compounds as catalysts may also suffer from yellow discoloration, as described in U.S. Pat. Nos. 4,131,601 and 4,482,700.
There is a need for a catalyst system for the synthesis of polyesters, in particular polyethylene terephthalate and its copolyesters, having increased catalytic activity, no or only minimal effect on the properties of the polyester, and reduced toxicity concerns.